Children with SOX2-deficiency develop anophthalmia/microphthalmia and display neurological impairment. Here we report an essential role for astroglial SOX2 in brain anomalies and neurological defects. Sox2-deficiency inhibited postnatal astrocyte maturation without affecting astroglial proliferation and population expansion. Mechanistically, we found that SOX2 directly bound to a cohort of astrocytic signature genes such as those involving in glutamate transport and that Sox2-deficiency remarkably reduced glutamate transporter expression and compromised astrocyte function of glutamate uptake. Behaviorally, astroglial Sox2-deficient mice developed hyperactivity in locomotion while their motor skills, social capability, and learning abilities were unaffected. We found that astroglial Sox2-deficiency results in elevated glutamatergic synapse formation and elevated excitability of striatal medium spiny neurons, which has been shown to trigger hyperactive locomotion. Our study provides new insights into the biological mechanisms underlying brain defects in children with SOX2 mutations and unveils a novel connection between astrocyte SOX2 and brain development and behavior.